Ign Module and Tach RPMs relationship
Thread Starter
Burning Brakes
Joined: Nov 2000
Posts: 1,012
Likes: 1
From: Bowie MD
Ign Module and Tach RPMs relationship
Hi there, thanks for looking at my post. I am experiencing really HIGH RPMs on my tach, but the engine doesn't seem to be going that fast. I have calulated the expected RPM (and what was normal for 14 years), with the 3.07 rear and automatic. But now I am getting in fourth gear really high numbers. Like 50 MPH is 4,000, and 60 is 5,000 and 70 is 6,000 and 80 is off the scale. Now my question is: does the Ignition Module (14 years old) have some resistor/chip/transistor/diode that would under heat and age cause a miss reading probably higher voltage to be sent to the Tach in the car. Will replacing the the Ign module fix this? Will installing a MSD 6AL box (that is sitting in a box on passenger seat) replace the Ignition module that may be bad and sending out bad tach info. Am I right in tracing down the problem to the Ign module? Any help or suggestions are appreciated. :flag
[Modified by corvette1990, 2:55 PM 3/14/2004]
[Modified by corvette1990, 2:55 PM 3/14/2004]
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Re: Ign Module and Tach RPMs relationship (corvette1990)
The ignition module generates pulses for the tach circuit in your cluster and they go through a tach filter. You can install a new module and see if that fixes it, but there are still the filter and the tach circuit in the cluster to test. The GM service manual is meant for their service technicians and they do not test down to the component level so no info is made available for testing components.
Drifting
Joined: Aug 2000
Posts: 1,521
Likes: 0
From: West Islip New York
Re: Ign Module and Tach RPMs relationship (corvette1990)
Try putting a bench tach on the car to see if you can isolate the problem to either the distrbiuter or the instrument cluster.
GM can put a tester on the circuit to check for proper operation of the cluster as well.
The ignition module signal for the tach is also the control for charging the coil - how's the car running?
GM can put a tester on the circuit to check for proper operation of the cluster as well.
The ignition module signal for the tach is also the control for charging the coil - how's the car running?
Thread Starter
Burning Brakes
Joined: Nov 2000
Posts: 1,012
Likes: 1
From: Bowie MD
Re: Ign Module and Tach RPMs relationship (1MoorTym)
The car seems to be running ok, except that it is scary to see the rpm guage wrap around at WOT before the auto changes gears. Maybe I can borrow a tach from a neighbor and run wire through window, to isolate problem as described above. Keep those cards and letters coming. Thanks....
Burning Brakes
Joined: Jan 2001
Posts: 1,169
Likes: 0
From: Manhattan
Re: Ign Module and Tach RPMs relationship (corvette1990)
I have an Accel ign. module and my tach is still 2k+ high. The tach started messing up long after i installed the new ign module. So, in my case, they are not related.
1990 zf6, fx3 coupe
1990 zf6, fx3 coupe
Thread Starter
Burning Brakes
Joined: Nov 2000
Posts: 1,012
Likes: 1
From: Bowie MD
Re: Ign Module and Tach RPMs relationship (6spd)
I took the suggestion and hooked up an old MSD Tach up to the battery (red& black wires). Then Plugged the white wire from the distributor (tach) into the MSD Tach. W0W. My Rpm's were right where they were supposed to be. I then hooked both up at same time and compared the two. The Dash Tach is way off by several thousand RPMs at speed. YIKES. This means that something is wrong in the dash? I never did see a tach filter. Just a white wire going from tach to the black hose leading to the fiewallI taced the white wire all the way to under the brake booster, where it enters the firewall. No sign of splices or a cylinder looking capacitor. I can't even imagine how much a dash R&r is let alone a new gauge set. AND I am not going to mount that ugly large MSD Tach on my steering column, with a water pump hose clamp. That and the flashlight looking thing looks ridiculous to me. LOL :lol:
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Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Re: Ign Module and Tach RPMs relationship (corvette1990)
If I'm reading your post correctly, you have discovered the problem. No tach filter. This site: http://my.execpc.com/C4/4A/rodney1/corvette/ Shows a filter, part number, 14020025 to fit 1984 - 1989. My Corvette parts book shows 14093115 to fit 1984 - 1990. My book stops at 1990. My book is old, so maybe his no. supersedes mine. I do wonder about the application discrepancy, though. Time to play Sherlock Holmes. Good luck, and...
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